OLED (Organic Electroluminescent Display) is one of the hotspots in today's Flat Panel Display field. Compared with LCD (liquid crystal display), OLED has advantages such as low consumption, low production cost, self-luminescence, wide viewing angle, fast responding speed and so on.
Unlike using a stable voltage to control brightness of the LCD, OLED is current-driven and it is required a stable current to control light emitting. There is ununiformity in the Threshold voltages Vth of the driving transistors in the pixel circuits of OLED due to manufacturing process and device aging, so that the current flowing through OLED element at each pixel dot varies, which results in ununiformity in the display brightness, thereby affecting the display effect of the whole image.
For example, in a 2T1C pixel circuit, as illustrated in FIG. 1, the circuit comprises one driving transistor T2, one switching transistor T1, and one storage capacitor Cs. When a scan line Scan selects a row, a low level signal is inputted from the scan line; the P-type switching transistor T1 is turned on and the voltage at the data line Data is written into the storage capacitor Cs. When the scanning of this row ends, the signal inputted from the scan line Scan becomes a high level, so that the P-type switching transistor T1 is turned off, and a gate voltage stored in the storage capacitor Cs causes the driving transistor T2 to generate a current to drive OLED, thus ensuring consecutive light emitting of OLED element within one frame. The formula of the saturation current of the driving transistor T2 is IOLED=K(VGS−Vth)2. As stated above, the threshold voltage Vth of the driving transistor T2 would drift due to the manufacturing process and device aging, so that the current flowing through OLED element at each pixel dot varies as the threshold voltage Vth of the driving transistor T2 varies, which results in the ununiformity in the image brightness.
In view of the above, in a known compensation method, the threshold voltage is written by discharging the driving transistor, so that the current flowing through OLED element at each pixel dot is independent of the threshold voltage Vth of the driving transistor. However, the OLED element exists in the discharge path of the driving transistor, and thus not only the characteristics and display lifespan of OLED, but also the accuracy of the written threshold voltage Vth of the driving transistor would be affected, thus worsening the compensation effect, and affecting the displaying of the whole image.